Coal has long been used as a fuel for combustion. Currently, such coals are typically combusted with pressurized air and oxygen in fluidized fuel combustors such as circulating fluidized-bed combustors (“CFBs”) and pulverized coal boilers (“PCB”). CFBs generally operate at relatively moderate temperatures of from about 760° C. to about 930° C., and utilize “coarser” fuel particles having a dp(50) typically ranging from about 3000 microns to about 6000 microns. PCBs operate at higher temperatures of from about 1300° C. to about 1700° C., and utilize “finer” fuel particles having a dp(50) typically ranging from about 100 to about 200 microns.
The combustion results in heat energy that can be used to generate steam (e.g., a steam boiler) for a variety of uses, including for driving turbines to generate electricity. The combustion also produces hot gases that can be used to drive a turbine to generate electricity. A cogeneration facility is one in which both electricity and steam are made in order to more efficiently utilize the energy release of combustion, with the steam often being used to drive a steam turbine for additional electricity generation.
Many coals, however, are economically unfeasible for such use due to contamination and physical properties. For example, coals that contain significant amounts of impurities, such as sodium and chlorine (e.g., NaCl), may actually be unusable in combustion processes due to the highly corrosive and fouling nature of such components, thus requiring pretreatment to remove such impurities. Gaseous effluent streams from the combustion of such coals can also be problematic, particularly if the coal is not sufficiently cleaned prior to combustion. See, for example, Tillman, Duong, Figueroa and Miller, “Chlorine in Solid Fuels Fired in Pulverized Coal Boilers-Sources, Forms, Reactions, and Consequences: A Literature Review”, Foster Wheeler A G, Presented at Fuel Quality Conference, Banff, Canada, Sep. 28-Oct. 3, 2008.
While washing can be used in an attempt to clean such coals, the washing requires large amounts of fresh water which may not be economically feasible in many regions of the world, particularly those regions (arid areas, for example) where fresh water is at a premium. In addition, washing creates a number of wastewater issues.
Typically the addition of such a pretreatment renders the use of sodium and/or chlorine contaminated coals economically unfeasible. It would, therefore, be desirable to find a way to more efficiently pretreat these contaminated coals to removed a substantial portion of at least the inorganic sodium and/or chlorine content, while minimizing fresh water usage.
“Low-rank” coals are typically softer, friable materials with a dull, earthy appearance. They are characterized by relatively higher moisture levels and relatively lower carbon content, and therefore a lower energy content. Examples of low-rank coals include peat, lignite and sub-bituminous coals. Examples of “high-rank” coals include bituminous and anthracite coals.
In addition to their relatively low heating values, the use of low-ranks coals has other drawbacks. For example, the friability of such coals can lead to high fines losses in the feedstock preparation (grinding and other processing) and in the combustion of such coals. Such fines must be managed or even disposed of, which usually means an economic and efficiency disadvantage (economic and processing disincentive) to the use of such coals. For very highly friable coals such as lignite, such fines losses can approach or even exceed 50% of the original material. In other words, the processing and use of low-rank coals can result in a loss (or less desired use) of a material percentage of the carbon content in the low-rank coal as mined.
Low-rank coals contaminated with sodium and/or chlorine are, therefore, generally considered economically unsuitable for any purpose.
It would, therefore, be desirable to find a way to efficiently process sodium and/or chlorine contaminated low-rank coals to make such coals more economically feasible for combustion uses, while reducing or even eliminating fresh water requirements, as well as potentially reducing fines losses in both the feedstock processing and ultimate conversion of such low-rank coal materials in various combustion processes.